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Avalanche Forecast

Archived

Feb 24th, 2020–Feb 25th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

Recent snow created widespread storm slabs which are likely to remain surprisingly touchy where they sit on surface hoar. Extra caution is recommended where the snow is wind affected, where it sits on a (sun) crust, or shady polar slopes especially near treeline.

Confidence

High - We have a good understanding of the snowpack structure and confidence in the weather forecast Uncertainty is due to extremely variable snowpack conditions reported through the region.

Weather Forecast

MONDAY NIGHT: Clear periods with light flurries possible. Temperatures -10 to -15 C. Light wind.

TUESDAY: Mix of sun and cloud with trace to a few cm of snow. Temperatures -5 to -10 C. Light southwest or west wind.

WEDNESDAY: Mix of sun and cloud with trace to a few cm of snow. Temperatures -5 to -10 C. Light to moderate gusty west wind. Freezing level rising to around 1500 m in the afternoon.

THURSDAY: Similar to Wednesday with light to moderate west wind. 

Avalanche Summary

Storm snow was touchy over the weekend with small (10 to 30 cm deep) natural and human triggered soft Storm Slab and Loose Dry avalanches.

A few sporadic large avalanches were reported over the past 10 days. Most recently, a large (size 3) deep persistent slab avalanche was remotely triggered last Sunday just outside the forecast region near Mt. Seven. Photos in the Mountain Information Network report show this avalanche was initiated by a skier causing a whumpf on a ridge that released a small wind slab on an east-facing alpine slope which then stepped down to the facets at the bottom of the snowpack. 

Snowpack Summary

Around 20 to 30 cm of recent snow covers a widespread layer of surface hoar (size 5-15 mm.) at all elevations. On southeast through southwest facing slopes this surface hoar likely grew on a thin sun crust which is almost a perfectly nasty combination. The middle of the snowpack is strong; however, the base of the snowpack contains basal facets which are responsible for occasional and hard-to-predict large Deep Persistent Slab avalanches in shallow rocky start zones. 

Terrain and Travel

  • Be alert to conditions that change with elevation and wind exposure.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.